The scattered disc

The scattered disc is a sparsely populated region beyond the Kuiper belt, extending from 50 AU to as far as 100 AU and further. Objects in this region have highly eccentric orbits and are often wildly inclined to the orbital plane of the major planets. Two of the first scattered disc objects (SDO) to be recognised are 1995 TL8 (at 53 AU from the Sun) and 1996 TL66 (at 83 AU). Other objects detected include 1999 TD10,

2002 XU93 and 2004 XR190 (at 58 AU). Some astronomers prefer to use the term 'scattered Kuiper belt objects' for objects in this region.

Many of the SDOs are doomed in the long term because, sooner or later, their highly eccentric orbits will carry them close to the giant planets to undergo more scattering. They may last a few million years or even 100 million years in their current orbits, but eventually Neptune will flip them nearer Uranus, Saturn or Jupiter. These planets will, in turn, fling them outward, far beyond the Kuiper belt and into the Oort cloud or out of the solar system entirely, or closer to the Sun (where they will become comets).

One of the major scattered disc objects is Eris (2003 UB313, previously known as Xena). The elliptical orbit of this body takes it to within 36 AU from the Sun and as far out as 100 AU. It was the discovery of this object in

2003 that prompted astronomers to refine the definition of a planet. If

Figure 13.11 Eris, the largest known scattered disc object, and its moon Dysnomia. (Photo: NASA)

Eris had been classed as a planet, there may have been as many as fifteen planets in the solar system. In the end, the IAU decided on a definition that excluded Eris and also Pluto as major planets, instead classifying them as plutoids.

Eris has a diameter of 2400 km, which makes it larger than Pluto. It is the largest object found in orbit around the Sun since the discovery of Neptune and its moon Triton in 1846. At times Eris is even more distant than the Oort cloud object Sedna (see below), and it takes more than twice as long to orbit the Sun as Pluto (560 years). In 2005, a near-infrared spectrograph on the Gemini Telescope in Hawaii showed the surface of Eris to be mainly methane ice. Methane ice suggests a primitive surface unheated by the Sun since the solar system formed. If Eris ever had been close to the Sun, the methane ice would have boiled off. The interior of the plutoid is probably a mix of rock and ice, like Pluto. The elliptical orbit of Eris is tilted at an angle of 45° to the orbital plane of the major planets. Eris has also been found to have a moon, named Dysnomia.

Figure 13.12 Orbital path of Eris (2003 UB313).

Eris is currently about three times Pluto's distance from the Sun, following an orbit that is about twice as eccentric and twice as steeply inclined to the plane of the solar system.